1. Field of the Invention
The present invention relates to apparatus for setting an identification threshold for use in identifying and reproducing serial data from a baseband signal received or automatically setting the DC level of a received baseband signal, and advantageously applicable to an optical signal receiver configured to receive, e.g. an intensity-modulated optical signal.
2. Description of the Background Art
An optical signal receiver of the type is conventional which receives from a transmitter, e.g. an optical pulse signal modulated in intensity with the logical level of data to be sent. In this type of optical receiver, the received optical signal is converted to an electric signal corresponding thereto, and then compared in level with an identification threshold in order to determine the logical level of data received.
It is a common practice with the optical signal receiver of the type referred to above to set the identification threshold at the center of a dynamic range between the peak and bottom levels of the received electric signal. This scheme is extensively used because of its simplicity and versatility. However, when the transmitter uses, e.g. an EA (Electro-Absorption) modulator, the crossing point of the received signal where the curves of a positive-going and a negative-going edge cross each other when occurring at the same timing is apt to fail to coincide with the center of the dynamic range. In such a case, the optimum threshold produced in the receiver is also shifted from the center. Further, it is likely that the optimum threshold is shifted from the center due to the dispersion of wavelength or polarization mode.
In light of the above, systems for automatically adjusting the threshold in accordance with the received signal have been proposed in the past. Japanese patent laid-open publication No. 265273/1996, for example, discloses a system including a number-of-errors detecting circuit configured to determine the degree of errors having occurred in serial data identified and reproduced. The degree of errors determined is reflected back to a new identification threshold. This kind of threshold setting system, however, needs a high-speed, sophisticated logic integrated circuit for detecting errors in serial data. It was therefore difficult to implement simple, low-cost apparatus for setting an identification threshold.